There's so many answers to that question, I'll only give you a portion of the answer. The good news is, I believe most authors use the Limiter plugin that helps normalize the volumes for single track audio. But if you don't, then you can wind up with any number of volumes based on where the author has set the volume and how the audio is rendered out upon track export.

Probably the worst offenders are going to be the multi-track ones. That's because you have to mix several different tracks, each having its own volume. And let me tell you, having just done a total of two multi-track authorings, I spent at least a full day trying to get the f&*^% volume right on the final build. But, truth be told, it's still low. Amping up the volume on every track doesn't really work, as it just leads to one blaring track and others low.

If you've got an overall solution that could help out, I'm all ears, as it would be nice to have them all the same, but we're not all working at one company that can ensure that happens.